The present invention relates in general to voltage regulation within integrated circuits (ICs), and in particular to a voltage regulated charge pump.
Many electronic systems require more than a single power supply voltage level for operation. For example, a non-volatile memory circuit that may require 5 volts as a primary source of power, often requires higher voltages (e.g., 10 to 15 volts) for programming and/ or erasing functions. Similarly, communication and networking system circuits often require voltages other than the primary supply voltage to, for example, meet certain interface specifications.
Depending on the power requirements of such secondary supply voltages, it is desirable to generate them internally from the primary power supply. This eliminates the need for additional externally provided power supplies. To this end, voltage multiplying or charge pump circuits have been developed to generate the higher voltages from the primary supply voltage.
Charge pump circuits take advantage of the charge storing capability of capacitors to, for example, double the level of a primary supply voltage by bootstrapping. A typical example of a charge pump circuit for use in communication circuits is disclosed in U.S. Pat. No. 4,797,899. There, a network of switches and capacitors operate to generate voltages twice that of the primary Vdd supply in both positive (+2.times.Vdd) and negative (-2.times.Vdd) directions.
In certain applications, such as in networking systems, the secondary voltage ("Vpp") generated by the charge pump is used to drive transmission lines in accordance to specific networking protocol such as V.28 (RS-232), V.35, RS449, EIA-530-A, X.21, etc. When driving transmissions lines under load conditions, current is drawn, and Vpp correspondingly decreases from a pre-determined level, for example, +12 Vdc. Restoring Vpp to its target level has been the focus of two common approaches to regulating charge pump output voltages.
One conventional approach adjusts the frequency controlling charge pump switches used to charge up a reservoir capacitor. In particular, when a decrease in output voltage is sensed by the regulation circuitry, the charge pump control logic responds by increasing the frequency of the charging of the capacitor. The increased rate of capacitor discharge means that more charge is injected into the charge pump's output, thus increasing the Vpp level. Once the output voltage reaches its targeted level under certain load conditions, the frequency decreases to it original clocking rate for Vpp regulation. In the event the output voltage increases beyond its targeted level, the control logic in turn would slow the clocking frequency to permit excess Vpp to bleed down.
A significant drawback to this approach is that the relatively high and variable frequencies associated with the charging and discharging of the reservoir capacitor results in radiated switching noise (i.e., electromagnetic radiation). As semiconductor device dimensions and operating voltage levels decrease, switching noise generated by charge pump regulation, according to this approach, contributes to device malfunction.
Another common approach to charge pump regulation is to use a low-dropout (LDO) linear post-regulator. An LDO regulator generally employs a shunt regulator, such as an NPN transistor, after the generated output voltage, but in parallel with the load. In operation, when there is a decrease in Vpp, due to temperature or load current, the shunting device draws less current than when Vpp is at its targeted level. Since less current is drawn by the shunting device, the voltage drop across the device increases until Vpp is restored to its pre-determined level.
An obvious drawback to this approach is that the output voltage generated must be higher than the actual Vpp used in the application. For example, in an application requiring only 12 Vdc, the charge pump is required to generate a higher secondary voltage, such as 16 Vdc. The additional voltage bolsters Vpp by increasing the voltage drop across the shunting device. Therefore, during normal and lightly loaded conditions (e.g., Vpp at target level), the current drawn by the shunting device (i.e., power dissipation) is unnecessarily wasted. Furthermore, as semiconductor technology advances and transistor dimensions decrease, lower operating voltages are required, for example, to prevent breakdown of gate oxides in CMOS devices. Therefore, generating a higher voltage than is necessary, increases the chance that a semiconductor device will be subject to damaging voltages.
Therefore, there is a need in the art for a circuit and a method for regulating a charge pump output voltage that neither radiates switching noise nor consumes more power than is necessary.